# Accurate mass and radius determinations of a cool subdwarf in an eclipsing binary

## TL;DR

This paper reports the discovery and precise measurement of a cool subdwarf in an eclipsing binary, providing valuable data to validate theoretical models of low-metallicity, low-mass stars.

## Contribution

It presents the first accurate mass and radius measurements of a cool subdwarf, enhancing understanding of low-mass, metal-poor stars and validating stellar models.

## Key findings

- Validated theoretical mass-radius-temperature-luminosity relations for low-metallicity stars.
- Provided precise measurements of a cool subdwarf's physical properties.
- Enhanced understanding of low-mass star evolution in the Galaxy.

## Abstract

Cool subdwarfs are metal-poor low-mass stars that formed during the early stages of the evolution of our Galaxy. Because they are relatively rare in the vicinity of the Sun, we know of few cool subdwarfs in the solar neighbourhood, and none with both the mass and the radius accurately determined. This hampers our understanding of stars at the low-mass end of the main-sequence. Here we report the discovery of SDSSJ235524.29+044855.7 as an eclipsing binary containing a cool subdwarf star, with a white dwarf companion. From the light-curve and the radial-velocity curve of the binary we determine the mass and the radius of the cool subdwarf and we derive its effective temperature and luminosity by analysing its spectral energy distribution. Our results validate the theoretical mass-radius-effective temperature-luminosity relations for low-mass low-metallicity stars.